*{
    padding: 0;
    margin: 0;
    font-size: 14px;
    font-family: Arial, Helvetica, sans-serif;
    -webkit-user-select: none;
}

#box{
    width: 500px;
    height: 300px;
    >span{
        width: 100px;
        height: 50px;
        display: block;
        line-height: 50px;
        text-align: center;
        background: blue;
    }
    >#mark{
        width: 500px;
        height: 250px;
        background: yellow;
    }
}